Stavridis told radio host John Catsimatidis on New York's AM 970 that Trump's goal of 2 percent gross domestic product (GDP) spending on defense for NATO allies was a "smart" target, chiding European allies for not spending as much as "promised" on defense.

“He is correct to put pressure on the allies to meet the 2 percent of GDP," Stavridis said.

"That is a smart goal, and it is one we should push the Europeans to achieve," he added. "While they don’t spend quite as much as they have promised, they do spend quite a bit."

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Still, Stavridis cautioned the president to maintain good relationships with U.S. allies.

"I don’t understand what Trump means [by threatening to rearrange U.S. troops in Europe]. We are in a treaty alliance, which means that we are obligated under our own law and international law to be part of this alliance," Stavridis said.

The former NATO commander urged Trump to consider how member nations responded when the U.S. was attacked on 9/11.

"Let’s remember that in the whole history of NATO, the only time it has activated itself on Article 5 … where it says an attack on one will be considered an attack on all, was 9/11," he added.

News outlets this week reported that Trump wrote to several world leaders, including German Chancellor Angela Merkel and Canadian Prime Minister Justin Trudeau, last month with a demand that the nations contribute more to the NATO alliance.

“It will, however, become increasingly difficult to justify to American citizens why some countries do not share NATO’s collective security burden while American soldiers continue to sacrifice their lives overseas or come home gravely wounded,” Trump reportedly wrote to Merkel.

The report on Monday came just one week before Trump is scheduled attend an annual NATO summit in Brussels.